Broadcast-addressed packets should never transit a router which is
working
properly by default, firewall or not.

You're quite correct. Note well, though, that 1122 is *host*
requirements, and very deliberately does not discuss *router*
requirements.

That's true, but the constraints on sending ICMP errors in section
4.3.2.7 of RFC 1812 are pretty much identical to those of section
3.2.2 in 1122. On the particular, limited topic of when to not
send ICMP errors the distinction between hosts and routers doesn't
matter much.
Dennis Ferguson